1. Field of the Invention
This invention relates to a color temperature information forming device for forming information on color temperature.
2. Description of the Related Art
White balance has been an important subject of signal adjustment for color television cameras. The ratio of three primary color components red (R), green (G) and blue (B) included in the illumination of an object to be photographed greatly varies with the kind of the illumination. The white balance adjustment is performed for varying the chrominance signals of the camera to have these primary color components in the ratio of 1:1:1. Inadequate white balance adjustment prevents the color from being reproduced with fidelity as it results in a bluish or reddish tone of the whole television picture.
The white balance must be readjusted every time the illumination of the object changes from one kind to the other. To meet this requirement, various automatic white balance adjustable apparatuses have been proposed for simple and accurate automatic adjustment. Among these apparatuses, some of compact TV cameras and particularly home video cameras has been proposed to be provided with a so-called external automatic follow-up white balance device. This device is arranged to detect the illumination of the object by means or a light receiving system disposed separately from an image pickup or sensing optical system and to adjust the white balance automatically according to the result of detection so that the operator can be relieved from paying attention to white balance adjustment .
According to these proposals, two or three different color components included in the external light of the object are detected by two or three photodetectors such as photo diodes or photo transistors arranged with filters disposed in front of them for these colors. The outputs of these detectors are logarithmically compressed. A difference between the compressed values is obtained and used for equivalently detecting the ratio among the color components of the illumination light. Them, the gains of the color chrominance signal amplifiers of an image pickup signal processing system is controlled in accordance with the detected ratio.
FIG. 1 of the accompanying drawings shows in outline an example of the above stated arrangement of the prior art. The illustration includes color filters 1 and 5; photo diodes 2 and 6; amplifiers 3 and 7; logarithmic amplifiers 4 and 8; and an adder 9. The color filters 1 and 5 have different spectral transmittance characteristics from each other. Light fluxes which pass through the color filters 1 and 5 come to the photo diodes 2 and 6. The outputs of the photo diodes 2 and 6 which correspond to the amount of the transmission light pass through the amplifiers 3 and 7 to be logarithmically amplified by the logarithmic amplifiers 4 and 8. The outputs of the amplifiers 4 and 8 is added up at the adder 9 with the output of the amplifier 8 alone inverted before arrival at the adder. The arrangement is thus capable of obtaining the ratio between the outputs of the photo diodes 2 and 6 as a color temperature detection output. Assuming that the color filters 1 and 5 are arranged to pass red and blue lights, the output of the adder 9 gives a ratio between the red and blue components of the incident light.
In another example of the prior art arrangement, three photo sensors are used with color filters for red, green and blue lights disposed in front of them. Then, a ratio between the red and green components and a ratio between blue and green components are obtained through the same arrangement as in the case of FIG. 1.
The prior art examples given above, however, have necessitated use of many additional circuit elements besides the photo diodes in obtaining, for example, the ratio between the red and blue components of the external light as shown in FIG. 1. Another shortcoming of the prior art devices of the above stated kind resides in that: Since the illuminance range of the illumination of the object is wide, the logarithmic amplifier is required to have a high degree of precision. Besides, they necessitate some correction circuit for temperature correction or the like. The prior art devices thus result in a complex and expensive arrangement.